Optical plastic elements are of late years in great demand on the ground of their good natures that they are lighter in weight than the glass elements, highly resistible to impact, they do not require surface glinding, are easily mass-produced, and able to give lenses which have non-spherical surface or surfaces. The optical plastic elements, however, have several disadvantages that they have low evenness in their surfaces, narrow selectivity in making refractive indices and high birefringence, which are derived from their manufacturing conditions and their inherent characteristics, so that they, in fact, have many limitations in their practical uses.
Particularly, in the field of fine optics where the optical elements are used for reading and writing of information by means of laser or the like, there is much necessity to improve the birefringence of the optical plastic elements.
As is well known to the art, the birefringence of the optical plastic element varies, in many cases, in accordance with their molding conditions as well as their inherent characteristics. To this point of view, the double refraction of the optical plastic element has been tried to be reduced by way of improving the molding conditions of the elements. However, those trials are less fruitful under the existing circumstances in that the fine optical elements are gradually coming into higher requirement and in that more strict reduction of the birefringence, which can not be fully met by any improvement of the molding conditions, must be accomplished. Therefore, the birefringence of the optical plastic element is now intended to be reduced by changing the plastic material to be used for the preparation of the optical plastic element.